1. Field of the Invention
The present invention relates to a leakage-sound detecting apparatus used to detect a water leakage place or spot in a water service pipeline.
2. Description of the Prior Art
As a method for detecting a water leakage spot in a water service pipeline, there has been generally practiced a method in which a single leakage-sound detecting apparatus is placed on the ground surface where the water service pipeline is embedded or two leakage-sound detecting apparatuses are secured to a fire hydrant or the like installed at a desired position of the water service pipeline so that a propagation sound of the water leakage-sound is detected by these leakage-sound detecting apparatuses to analyze correlative waveforms outputted, thereby detecting the water leakage spot.
In the leakage-sound detecting apparatuses heretofore used, a piezoelectric element is encased in a metal casing. This has its object to detect sounds of a wide range of frequency components. Therefore, there is a flat sensitivity characteristic up to a high frequency component such as several kHz, in general.
With the recent intense trend of replacement of water service pipes, that is, replacement of conventional metal pipes with plastic pipes such as vinyl chloride, polyethylene chloride and the like in accordance with one way of preparation of the pipeline service by the water service undertaking body, higher efficiency of water-leakage detection in connection with plastic pipes has also been eagerly demanded.
However, the water leakage sound propagating along the plastic pipes, e.g. the sound of high frequency component in excess of 500 Hz becomes greatly attenuated as compared to metal pipes, and only the sound of frequency component from scores of Hz to hundreds of Hz results.
Further, the conventional leakage-sound detecting apparatuses have a wide band of sensitivity characteristic, and therefore detect even foreign noises other than the water leakage sound to be detected.
For this reason, the conventional leakage-sound detecting apparatuses are hard to efficiently detect the water-leakage propagation sound whilst are apt to detect the foreign noises, thus making it extremely difficult to perform higher efficiency of detection of water leakage in connection with the plastic pipes.